1. Field of Invention
The present invention relates to an LLC series resonant converter and the driving method for the synchronous rectification power switches, and particularly to an LLC series resonant converter adopting a frequency adjustment controller to provide driving signals and the driving method of the synchronous rectification power switches thereof.
2. Description of Related Art
FIG. 1 is a prior art circuit configuration of a widely used LLC series resonant converter (SRC) having a half-bridge structure. Here, LLC represents a circuit composed by two inductors and a capacitor. It includes a half-bridge circuit (110) formed by a pair of power switches S1 and S2 for driving a resonant network (120). The resonant converter has 3 resonant parameters including a series resonant inductor Ls, a series resonant capacitor Cs and a magnetizing inductor Lm of a transformer TX (not shown in FIG. 1). The series resonant inductor Ls can also be formed by leakage inductance from the transformer TX. Such three resonant parameters determine two characteristic resonant frequencies fs and fm shown in following:
                    fs        =                  1                      2            ⁢            π            ⁢                                                            L                  s                                ·                                  C                  s                                                                                        (        1        )                                fm        =                  1                      2            ⁢            π            ⁢                                                            (                                                            L                      s                                        +                                          L                      m                                                        )                                ·                                  C                  s                                                                                        (        2        )            
The input terminal of the LLC series resonant converter is a DC voltage Vin. The transformer TX isolates the half-bridge circuit (110) and the resonant network (120) from a rectification circuit (130) by a primary side winding np and two series secondary sides winding ns1 and ns2, coupled in series.
The rectification circuit (130) includes a pair of rectification diodes D1 and D2 connected to an output capacitor Co. The cathodes of D1 and D2 are connected to the anode of capacitor Co. The anode of D1 is connected to the positive terminal of the secondary side coil ns1, whereas the anode of D2 is connected to the negative terminal of the secondary side coil ns2. A common connection node between the coils ns1 and ns2 is a ground of an output voltage Vo.
The power switches S1 and S2 of the LLC series resonant converter operate under the condition of equal pulse width, that is, in the condition of 50%. A frequency adjustment controller 140 is therefore demanded because the adjustment of the output voltage is obtained by changing the operation frequency.
LLC series resonant converter has the characteristics that proper parametric design and operating ranges can guarantee the power switches of a primary side bridge circuit to be operated under a zero voltage switching (ZVS) condition, and at the same time causing the rectification switches of a secondary side to perform as zero current switching (ZCS).
FIG. 2 is a schematic diagram of the operating situation time sequence of the LLC series resonant converter shown in FIG. 1, when the operating frequency f of power switches S1 and S2 satisfies the formula offm≦f≦fs  (3),wherein Vg,S1 and Vg,S2 respectively refer to the driving signals for the power switches S1 and S2.
At time of t0, because the primary side current ir is opposite to a reference direction, the power switch S1 opens under the ZVS condition. During the interval between t0 and t1, the rectification diode D1 is conducted with current, therefore the voltage on the magnetizing inductor Lm is constant and does not take part in the resonance, so that the magnetizing current im increases linearly. Referring to FIG. 2, due to the resonance between Ls and Cs, current iD1 through D1 appears in a quasi-sine waveform.
At time of t1, because the switching period is longer than the resonant period between Ls and Cs, ir descends down to im before the shutting off of S1, so D1 is off at this moment. Cs, Ls and Lm take part in the resonance. For the purpose of simplifying analysis, supposed to be the condition of Lm>>Ls, then the ir is approximately a straight line.
At time of t2, S1 is turned off and S2 is conducted. At time of t3, S2 is switched on under ZVS condition.
During the intervals from t3 to t4 and from t4 to t5, similar process can be analyzed out. The operation status and current waveform iD2 similar to D1 also happen to the rectification diode D2. iD1 and iD2 compose the output rectification current irec.
If the LLC series resonant converter operates at switch frequency fs, the dead time of output rectification current irec, in which both rectification diodes D1 and D2 are non-conducting during the dead time, disappears and rectification current irec has a quasi-sine absolute value waveform.
In order to operate in a wide range of input voltage and output load, the LLC series resonant converter in practical operation is usually operating under the condition that the dead time the irec is zero. As, at this time, the shutting off action of the rectification diode D1 or D2 always happens before the power switches S1 or S2 are off, the conducting pulse widths of D1 and D2 are smaller than those of S1 and S2. Nowadays, it becomes popular to adopt a power metal oxide semiconductor field effect transistor (MOSFET) to substitute rectification diodes D1 and D2 operating as synchronous rectification power switches. In this manner, the driving pulse of the synchronous rectification power switches has to be off when the backward current flowing from the source to the drain descends down to zero, that is, non-conducting state during the dead time of the irec. Otherwise, the output terminal of the converter would transport power to the primary side of the converter, and the circuit would not be able to operate properly and safely. Accordingly, the driving signals of power switches S1 and S2 cannot be used to obtain the driving signals for synchronous rectifiers.
Moreover, the coil of the transformer TX could be used to obtain the driving signals for the synchronous rectifier power switches, since during the dead time of irec, the voltage of secondary side coil is not zero, but is the resonant voltage on Lm.
Therefore, better technique is demanded to solve the above problems to provide proper driving signals, so that the power MOSFET can replace the rectification diodes and the synchronous rectification in the LLC series resonant converter can be implemented, and it can guarantees the safe and reliable operation under any operation condition.